Surface acoustic wave (SAW) devices have been regularly used for the passive retrieval of information via a wireless retransmission of interrogative radio frequency (RF) signals. This information could be in the form of a modulated code embossed into the geometry of the device for use as identification device (RFID) tags or information pertaining to a sensor that is attached as a load impedance. These thin film devices require no battery for certain operations as they selectively reflect an interrogating RF pulse back to an interrogating transceiver with the information adapted into the RF signal.
A SAW RFID tag or sensor typically has a suitable antenna attached to the input/output inter-digital transducer (IDT) of the SAW device and receives an RF pulse sent from the interrogation transceiver located either line-of-sight or non-line-of-sight a few metres away. This RF pulse is transformed by the IDT into an acoustic wave and propagates towards other IDTs adjacent to the originating IDT placed on the same piezoelectric substrate. The other adjacent reflective IDT is then configured either as a modulated structure for an RFID application or has a measuring load attached for an application as a sensor.
SAW RFID tags are ideally suited for people or asset tracking. The tags are inconspicuously small, require no battery for certain operations and can be easily read by interrogating circuitry at distances of a few metres in either a line-of-sight or non-line-of-sight path. Generally 32, 64 or even 96 bits are encoded into the geometries of these devices to provide unique identification for up to billions of items. Low cost is one of the major parameters associated with RFID tags, as most of them will be disposable and non-recoverable.
SAW sensors have a wide range of applications which include automotive (torque, proximity radar and tire pressure), medical (biosensors) and industrial (temperature, pressure, humidity mass and vapour) applications.